Christoph Dobraunig
Lamarr Security Research, Graz, Austria

Lorenzo Grassi
Digital Security Group, Radboud University, Nijmegen, Netherlands

Lukas Helminger
IAIK, Graz University of Technology, Graz, Austria; Know-Center GmbH, Graz, Austria

Christian Rechberger
IAIK, Graz University of Technology, Graz, Austria

Markus Schofnegger
IAIK, Graz University of Technology, Graz, Austria

Roman Walch
IAIK, Graz University of Technology, Graz, Austria; Know-Center GmbH, Graz, Austria

Keywords: homomorphic encryption, hybrid homomorphic encryption, Pasta, SEAL, HElib, TFHE

Abstract

The idea of hybrid homomorphic encryption (HHE) is to drastically reduce bandwidth requirements when using homomorphic encryption (HE) at the cost of more expensive computations in the encrypted domain. To this end, various dedicated schemes for symmetric encryption have already been proposed. However, it is still unclear if those ideas are already practically useful, because (1) no cost-benefit analysis was done for use cases and (2) very few implementations are publicly available. We address this situation in several ways. We build an open-source benchmarking r framework, we explore properties of the respective HHE proposals. It turns out that even medium-sized use cases are infeasible, especially when involving integer arithmetic. Next, we propose Pasta, a cipher thoroughly optimized for integer HHE use cases. Pasta is designed to minimize the multiplicative depth, while also leveraging the structure of two state-of-the-art integer HE schemes (BFV and BGV) to minimize the homomorphic evaluation latency. Using our new benchmarking environment, we extensively evaluate Pasta in SEAL and HElib and compare its properties to 8 existing ciphers in two use cases. Our evaluations show that Pasta outperforms its competitors for HHE both in terms of homomorphic evaluation time and noise consumption, showing its efficiency for applications in real-world HE use cases. Concretely, Pasta outperforms Agrasta by a factor of up to 82, Masta by a factor of up to 6 and Hera up to a factor of 11 when applied to the two use cases.